1. Field of the Invention
This invention relates to downhole seismic services and more particularly to a method for deployment, mounting and coupling of motion sensors and sources downhole.
2. Description of the Related Art
Seismic sources and sensors are often deployed in wellbores for a variety of oilfield operations, including monitoring of injection well operations, fracturing operations, performing xe2x80x9cseismic-profilingxe2x80x9d surveys to obtain enhanced subsurface seismic maps and monitoring downhole vibrations. Such operations include slim-to large-diameter boreholes, vertical to horizontal wells, open and cased holes, and high pressure and high temperature wells. Downhole sensors are sometimes utilized in combination with other logging services, either wireline, coiled tubing-conveyed, or with pipe to provide additional reservoir information.
Seismic sensors deployed in wellbores are particularly useful to monitor fracturing and injection well operations, to generate cross-well information and to obtain seismic measurements over time, to obtain enhanced subsurface maps and to improve reservoir modeling. However, the majority of seismic data gathering is accomplished by wireline methods or by deploying seismic sensors such as geophones on coiled tubing or production pipe. Multi-component geophones are usually preferred for such applications. Multi-component geophones sense motion in one or more directions. An example is the classical three (3) component geophone which detects particle motion in three mutually orthogonal directions (x, y and z directions).
An inherent problem with commonly utilized deployment methods for motion sensors in wellbores is the presence of high amplitude vibrations. The high amplitude vibrations may be due to the motion of the wireline or tubing used to carry these sensors in the wellbore. Even when these motion sensors are attached to the tubing, the sensors are subjected to substantial undesired motion due to the movement of the tubing in the wellbore or other operating factors. Ideally, a sensor deployment system should be free of all motion, thus enabling the sensors to accurately detect motion due to induced acoustic signals. Presence of spurious motion associated with movement of the tubing in the wellbore can significantly reduce the signal to noise ratio and mask the desired seismic signal in a high amplitude noise field.
Thus there is a need for a method and apparatus that reduces motion and noise associated with movement of tubing in the wellbore.
Geophones which are rigidly coupled to the wellbore, particularly in production wells, can provide high fidelity signals, i.e., with high signal to noise ratio. Such sensors are less likely to resonate. Distributed sensors can provide measurements useful for a number of applications, including monitoring of fracturing, seismic-profiling surveys, cross-well tomography and monitoring of injection operations.
Directly coupling of the seismic receivers to the borehole, wherein the coupling force is substantially greater than the radial and axial force on the sensor due to operating conditions, provides signals with the desired high fidelity. Inadequate or defective coupling, however, induces distortion of seismic wavelets, including data amplitude loss, phase change and bandwidth reduction. Downhole ambient noise can swamp recorded data. It is also well known that the quality of the data detected by the motion sensors improves with the use of receiver arrays (distributed sensors) and with the acquisition of redundant data.
Seismic sources are also placed in wellbores to induce acoustic waves in the formation for the kinds of operations described above with respect to receivers. Vibratory sources are often used as the acoustic sources. Directly coupling of the acoustic source in the wellbore greatly impacts the amount of energy transmitted into the formation. Smaller sources can be utilized with direct coupling because the energy loss between the source location and the receiver(s) is reduced.
In one aspect, the present invention provides a method of placing acoustic devices in wellbores. The method includes providing a tubing with at least one anchoring device in the wellbore, the anchoring device being extendable to the wellbore to exert a predetermined force on the wellbore, and attaching at least one acoustic device to at least one anchoring device; placing the tubing in the at least one acoustic device attached to at least one anchoring device in the wellbore; and setting the anchoring device to extend to the wellbore to exert a predetermined force on the wellbore, thereby coupling the acoustic device to the wellbore. The acoustic device is attached to the anchoring device so that the acoustic device would be located in an annulus between the tubing and the wellbore when the tubing is placed in the wellbore. Multiple spaced-apart acoustic devices may also be used. For example, spaced-apart acoustic detectors may be used in the wellbore, forming an array of detectors for detecting seismic wavelets.
The acoustic device used may be any one of a plurality of geophones, at least one source; or a combination including at least one acoustic source and at least one acoustic detector. The anchoring device may be any one of a hook-wall packer, an inflatable packer, a tubing anchor, a tubing hanger, a whipstock packer, a sump packer, a tubing centralizer, or a mechanically expandable elastomeric packer.
Thus, the present invention provides a wellbore for oilfield operations wherein the wellbore includes a tubing with an annular space between the tubing and the wellbore; and at least one anchoring device disposed on an outer surface of the tubing. The anchoring device extends to and exerts a predetermined force on the wellbore. An acoustic device is attached to the anchoring device prior to the placement of the tubing in the wellbore. When the anchoring device is set in the wellbore, it couples the acoustic sensor with the wellbore. A line attached to the acoustic device provides power (electrical, optical, hydraulic, etc.) to the acoustic device. This line also provides data communication and control between the acoustic device and surface control units, such as a processor, which may be a computer or another data processing and control unit such as a micro-processorbased unit.
Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.